The present invention relates generally to methods and apparatus for reproducing images and alphanumeric characters, and more particularly to a thermal inkjet drop generator, printhead construction, and the respective method of operation.
The art of inkjet printing technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ inkjet technology for producing hard copy printed output. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Inkjet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy Devices, chapter 13 (Ed. R. C. Durbeck and S. SherT, Academic Press, San Diego, 1988).
A thermal inkjet printer for inkjet printing typically includes one or more translationally reciprocating print cartridges in which small drops of ink are formed and ejected towards a medium upon which it is desired to place alphanumeric characters, graphics, or images. Such cartridges include a printhead having an orifice member or plate that has a plurality of small nozzles through which the ink drops are ejected. Adjacent to the nozzles are ink firing chambers, in which ink resides prior to ejection through the nozzle. Ink is supplied to the ink-firing chambers through ink channels that are in fluid communication with an ink supply, which may be contained in a reservoir portion of the print cartridge or in a separate ink container spaced apart from the printhead.
Ejection of an ink drop through a nozzle employed in a thermal inkjet printer is accomplished by quickly heating a volume of ink within the adjacent ink firing chamber with a selectively energizing electrical pulse to a heater resistor positioned in the ink firing chamber. At the commencement of the heat energy output from the heater resistor, bubble nucleation generally commences at locations of dissimilarities in the ink liquid or at defect sites on the surface of the heater resistor or other interface surfaces (heterogeneous nucleation). It is well known that heterogeneous nucleation of an ink vapor bubble is favored to occur energetically at interfaces. Although it is possible to promote homogeneous nucleation, it is not possible to do so in the absence of heterogeneous nucleation occurring at the interface between the ink and the contact surface where heat transfer occurs. If the location of these nucleation sites is not optimized, bubble formation will occur randomly or at various uncontrolled sites within the ink firing chamber. Therefore, although one may wish to drive the process to homogeneous nucleation on the heating surface of the structure, it is heterogeneous nucleation which occurs due to its reduced energy requirement at the high energy interface. The rapid expansion of the ink vapor bubble forces ink through the nozzle. Once ink is ejected, the ink-firing chamber is refilled with ink from the ink channel and ink supply.
The energy required to eject a drop of a given volume is referred to as "turn on energy". The turn-on energy is a sufficient amount of energy to form a vapor bubble having sufficient size to eject a predetermined amount of ink through the printhead nozzle. Following removal of electrical power from the heater resistor, the vapor bubble collapses in the firing chamber in a small but violent way. Components within the printhead in the vicinity of the vapor bubble collapse are susceptible to fluid mechanical stresses (cavitation) as the vapor bubble collapses and ink crashes into the ink firing chamber components between firing intervals. The heater resistor is particularly susceptible to damage from cavitation. A thin hard protective passivation layer is typically applied over the resistor and adjacent structures to protect the resistor from cavitation. The passivation layer, however, tends to increase the turn-on energy required for ejecting droplets of a given size. Another layer is typically placed between the cavitation layer and the heater resistor and associated structures. Thermal inkjet ink is chemically reactive, and prolonged exposure of the heater resistor and its electrical interconnections to the ink will result in a chemical attack upon the heater resistor and electrical conductors. A hard non-conductive passivation layer is disposed over the heater resistor to provide this protection from the ink. The cavitation layer and the passivation layer can be thought of, in concert, as a protective layer. Significant effort has been expended in the past to protect the heater resistor from cavitation and attack, including the separating of the heater resistor into several parts and leaving a center zone (upon which a majority of the cavitation energy concentrates in a top firing thermal inkjet firing chamber) free of resistive material.
Significant effort is also expended in improving print quality. Print quality has become one of the most important considerations of competition in the color inkjet printer field. Since the image output of a color inkjet printer is formed of individual ink drops, the quality and fidelity of the image is ultimately dependent upon the quality of each ink drop and its placement and arrangement as a dot on the printed medium.
One source of reduced print quality is improper ink drop volume. It is known that drop volumes vary with the printhead substrate temperature because the properties that control it vary with temperature: the viscosity of the ink itself and the amount of ink vaporized by a heater resistor when driven by a given electrical printing pulse. Changes in drop volume also cause variations in the darkness of black text, variations in the contrast of gray-scale images, as well as variations in the chroma, hue, and lightness of color images. In a printing system that employs a limited number of color inks, the chroma, hue, and lightness of a printed color depends upon the volume of all the primary color drops that create the printed color. If the printhead substrate temperature increases or decreases as a page of media is printed, the colors at the top of the page can differ from the colors at the bottom of the page. Additionally, when at room temperature, a thermal inkjet printhead must eject drops of sufficient size to form satisfactory printed text or graphics. However, printheads that meet this performance requirement can eject drops containing excessive amounts of ink when the printhead substrate is warm. Excessive ink degrades print quality by causing feathering of the ink dots, bleeding of the dots having different colors, and cockle and curling of the medium. In addition, different print media, i.e., plain paper, special paper, or transparency material requires different ink drop volumes for optimum performance. Controlling the ink drop volume depending upon the above conditions helps to eliminate these problems and improve print quality.
Generally, the drop volume from an inkjet printer printhead can be adjusted by varying the drop generator physical geometry (changing the heater resistor size and nozzle orifice size), varying the ink refill speed (changing the backpressure, ink filter fluid resistance, and ink feed channel restrictions), varying the size and strength of the vaporization bubble (adjusting ink temperature, nucleation surface heating rate, and nucleation surface roughness and cleanliness), and varying fluidic response such as ink viscosity (which is also a function of ink temperature). A related method of adjusting drop volume is that of ejecting multiple smaller droplets to deposit neighboring or overlapping dots on the printed medium. The foregoing factors can be divided into two categories: factors that can be dynamically changed by operation of the printer and factors that are fixed design parameters. Of the above factors, only temperature, nucleation surface heating rate, and multiple droplet expulsion can be dynamically adjusted by the printer.
Printhead temperature control has been discussed in, for example, U.S. Pat. No. 5,673,069 "Method and Apparatus for Reducing the Size of Drops Ejected from a Thermal Ink Jet Printhead". Variation in the electrical pulse width supplied to the heater resistor, thereby affecting nucleation surface heating rate, will produce a variable drop volume proportional to the pulse width. U.S. Pat. No. 5,726,690, "Control of Ink Drop Volume in Thermal Inkjet Printheads by Varying the Pulse Width of the Firing Pulses" discloses a method for doing so. Others have shown that printheads could be constructed with a protective layer having a thickness gradient. See U.S. Pat. No. 4,339,762, "Liquid Jet Recording Method". This gradient provides a positional variation in the point of bubble nucleation relative to the applied electric potential. When utilized in a system that ejects ink drops parallel to the plane of the heater resistor, the volume of the drop of ink can be made a function of the location of nucleation on the heater resistor and therefore a function of the applied electric potential. Multiple droplet deposition, such as that described in U.S. Pat. Nos. 4,967,203, "Interlace Printing Process "; U.S. Pat. No. 4,999,646, "Method for Enhancing the Uniformity and Consistency of Dot Formation Produced by Color Ink Jet Printing"; and U.S. Pat. No. 5,583,550, "Ink Drop Placement for Improved Imaging", have the disadvantage of decreasing the throughput of the printer.
The efforts of others notwithstanding, a variable drop mass having good control of ejected drop direction in a thermal inkjet printer printhead has not been readily achieved. It is highly desirable, at least for reasons of alphanumeric character quality and color image fidelity, that a dynamic selection of ink drop mass be made available for an inkjet printer without excessive cost, reduction in throughput, or degraded directionality of drop ejection.